MCHAS Modernising Children’s Hearing Aid Services Acoustic Feedback Reduction

MCHAS Modernising Children’s Hearing Aid Services Introduction  Acoustic Feedback Principle  Acoustic Feedback Reduction Strategies  Accessing Acoustic Feedback Tests  Summary

MCHAS Modernising Children’s Hearing Aid Services Acoustic Feedback Principle Input signal Feedback signal Output signal ( In phase) Input signal Feedback signal Feedback problems = GN Resound

MCHAS Modernising Children’s Hearing Aid Services  Feedback occurs when: –the ear mould is damaged or a poor fit –if the ear canal is occluded with wax –when the hearing aid is turned up too much.  Feedback reduction algorithms try to ensure that the hearing aid is never turned up too much by: –the clinician –the aid wearer –automatically by the hearing aid itself.

MCHAS Modernising Children’s Hearing Aid Services Feedback Management Strategies Desired Dillon Frequency (Hz) Hearing aid gain (dB)

MCHAS Modernising Children’s Hearing Aid Services Desired Gain Reduction Strategies Volume decreased Dillon 2001

MCHAS Modernising Children’s Hearing Aid Services Gain Reduction Strategies Dillon 2001

MCHAS Modernising Children’s Hearing Aid Services Gain Reduction Strategies  The maximum gain that can be achieved in different frequency regions is limited.  Need to know what the maximum amount of gain (in each frequency) is, before feedback will occur. This safe value can be set by: –the clinician at the time of fitting –the fitting software at the time of fitting (in-situ measurements) –the hearing aid whenever the hearing aid is worn

MCHAS Modernising Children’s Hearing Aid Services  Static Notch Filters –Fixed filters which limit the gain only around the frequencies where feedback oscillation is occurring. –Filters can only be changed by running a new feedback check. –Limitation: Feedback oscillation frequency does not remain fixed over time Gain Reduction Strategies

MCHAS Modernising Children’s Hearing Aid Services Gain Reduction Strategies  Active Notch Filters -Also called Roving Notches -HA actively monitors it’s own output for feedback and creates a notch filter at the frequency it detects feedback. -Can implement two or three spikes -More effective than static notches but are limited and may be slow to adapt (Hayes, 2003)

MCHAS Modernising Children’s Hearing Aid Services Feedback Cancellation  Most sophisticated strategy as there is no gain reduction.  HA detects signal leaking back to the microphone and automatically cancels this signal by adding some equal but opposite signal.

MCHAS Modernising Children’s Hearing Aid Services External leakage path + - Internal feedback path Feedback Cancellation Dillon 2001

MCHAS Modernising Children’s Hearing Aid Services Feedback Cancellation  Highly adaptive  Can control several feedback spikes simultaneously without sacrificing sound quality  However, battery life, sampling time and processing power are key to its effectiveness (Hayes, 2003)

MCHAS Modernising Children’s Hearing Aid Services Feedback Cancellation  Feedback cancellation is also known as ‘search & destroy’ and can allow up to 10 dB of extra gain before feedback.  Limitation: Some musical tones (highly periodic) and simple test box signals may also be suppressed Max gain with DFS 2. Max gain without DFS Taken from actual Danalogic 163D fitting with 2 mm vent (GN Resound)

MCHAS Modernising Children’s Hearing Aid Services Accessing Feedback Tests  Active gain reduction strategies –Phonak Supero the active notch filter is deactivated during the fitting process and in the music program. –In the Siemens Prisma 2M and DSP notch filters are removed after 5 mins to see if feedback is still present. –Starkey Strata there is an automatic gain reduction in the high channel.  Feedback Cancellation –Danalogic 283D use this strategy. The ‘Digital Feedback Suppression’ (DFS) can be deactivated in the software. –Spirit II Direct and Power

MCHAS Modernising Children’s Hearing Aid Services Accessing Feedback Tests.  All feedback tests can be accessed by clicking on a software icon or going into one of the ‘pull down’ menus.  All tests require noise free environments.  There may be a number of different options available. Instructions normally accompany any procedure but if you have difficulties try reading the instructions in the help menu.

MCHAS Modernising Children’s Hearing Aid Services SPIRIT II DIRECT & POWER

MCHAS Modernising Children’s Hearing Aid Services DANALOGIC 283D

MCHAS Modernising Children’s Hearing Aid Services PRISMA 2M & DSP  Optimize will insert static notch if necessary  Adaptive sets up active notch filter(s)  If feedback occurs aid checks for presence after 30 secs and 5 minutes. Notch(es) will be removed if feedback signal no longer present.  Can implement one notch per channel (up to 4)

MCHAS Modernising Children’s Hearing Aid Services STRATA A312

MCHAS Modernising Children’s Hearing Aid Services Phonak Aero & Supero

MCHAS Modernising Children’s Hearing Aid Services NHS DSP Hearing Aid FB Strategies Hearing Aid Limits gain in channels Static Notch Filter Adaptive Notch Filter Phase Cancellation Phonak Aero 211 & 311 AZ Phonak Supero Oticon Spirit II Direct & Power Siemens Prisma 2M & DSP Starkey Strata A312 * Active high channel gain reduction * Active high channel gain reduction Danalogic 283D

MCHAS Modernising Children’s Hearing Aid Services Thank-you Questions?